Brandis due to be London-bound next month

Former attorney-general George Brandis is expected to be the new UK high commissioner in time to welcome Malcolm Turnbull to London.

DFAT Secretary Frances Adamson told a Senate hearing on Thursday that Mr Brandis would replace Alexander Downer ahead of the Commonwealth Heads of Government Meeting on April 19.

Mr Downer's three-year team was due to expire in May last year.

Asked by Labor to confirm whether the position requires a "big beast" - as Mr Brandis had said Malcolm Turnbull was seeking - Ms Adamson joked she very happily served in the role for six months and wouldn't regard herself as one.

"It could be done by people of a range of sizes I would say," she said.

But Ms Adamson did acknowledge one of the key qualities the receiving country expects is that the head of mission will enjoy the absolute confidence of Australia's prime minister.